High production machine tools require material-handling systems which are capable of precisely feeding stock to the machine tool at a relatively high rate or else the production capacity of the machine tool is not fully utilized. A complete material-handling system should be capable of not only storing and conveying work pieces to the general area of the machine tool, but also positioning the work piece precisely and accurately in a registered or indexed position with respect to the machine tool for performance of a machining operation.
Prior production material-handling systems have been devised which are capable of delivering material to be processed at a relatively high rate to the machine tool, but registration of the work piece at the tool has lacked precision. The problem of positioning the work piece with respect to the machine tool is further exacerbated by the frequency with which the material or work pieces are irregularly formed. Crooked, bent or warped material, or pieces which have surface irregularities or burrs and ragged end cuts, will substantially influence the precision with which conventional material-handling apparatus will register a work piece in a machine tool for machining.
Typical of the prior art material-handling systems for machine tools are the power roller conveyors which are employed to feed bar stock to horizontal band saws. These systems employ chain or gear driven rollers which advance the bar stock toward the work area at between about 9.15 to about 18.3 meters per minute (30 to 60 feet per minute). The bar stock is usually advanced past the machine tool until it engages a stop. In some systems, the power rollers are slowed down prior to impact of the stop by the work piece, and they slip under the part and try to force the part against the stop.
While theoretically this approach will index the bar stock with respect to a machine tool, in practice, the registration of the bar with respect to the cutting tool is very poor. As will be appreciated, some bar stock will have considerable mass. Upon impact with virtually any stop, there will be deflection of the stop and/or rebound of the bar stock, and the mass of the stock will cause a variation in the amount of deflection. Additionally, the drive train for power rollers is inherently very loose and sloppy. It is virtually impossible, therefore, to accurately stop the power rollers, as they tend to coast and take out slack in the drive components. Moreover, the bar stock is not in any way positively secured to the power rollers, but merely rests on the same and is conveyed by means of friction and gravity.
The net result of conventional power roller material-handling systems has been that deflection of the stop, rebound of the material, slack in the power rollers and frictional sliding all contribute to relatively inaccurate placement of the bar stock with respect to the machine tool. Warpage, bending, cut-off spurs and the like exaggerate the inaccuracies.
Such prior material-handling systems can be tolerated in the industry for some applications in which machining accuracy is not critical. Alternatively, a production worker can be used to assist in the indexing of the stock so as to enhance the precision with which it is placed under the machine tool. Even the use of hand labor, in addition to its extra expense, may not be entirely satisfactory in achieving the precision which is desirable in the industry, namely, a variation of no more than about .+-.3.8 millimeters (0.015 inches) and preferably no more than about 0.635 millimeters (0.0025 inches).